Optical disc recording and/or reproducing apparatus and aberration adjustment method

ABSTRACT

The astigmatic aberration in an optical system is to be adjusted. Specifically, in an optical disc device on which can be selectively loaded a first optical disc having a first index of double refraction or a second optical disc having a second index of double refraction larger than said first index of double refraction, and which includes a liquid crystal device  31  between a light source  11  and an objective lens  15  converging a light beam radiated from said light source on the optical disc  2  loaded on the optical disc device, the voltage applied to the liquid crystal device  31  is adjusted to correct the coma aberration or the astigmatic aberration for the first disc or the second disc, respectively.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division and claims the benefit of priority under35 U.S.C. §120 from U.S. application Ser. No. 10/026,740, filed Dec. 27,2001, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and an apparatus for recording and/orreproducing information signals for a plurality of sorts of opticaldiscs having reciprocally different values of track pitch of therecording tracks and hence differential recording densities. Thisinvention also relates to a method for adjusting the aberration of thepresent optical disc recording and/or reproducing method and apparatus.

2. Description of Related Art

In an optical disc used up to now as a recording medium for informationsignals, attempts have been made for raising the recording density. Forexample, in the case of a magneto-optical disc having a diameter ofapproximately 65 mm, proposals have been made for reducing the trackpitch of the recording track carrying the information signals from 1.6μm to 0.95 μm to increase the recording density by a factor ofapproximately five.

For recording information signals on the magneto-optical disc, havingthe reduced track pitch as described above, and for reproducing therecording information signals, the spot diameter of the light beamscanning the recording track formed on the magneto-optical disc needs tobe of a smaller value. The reason is that, if the spot diameter of thelight beam becomes larger than the track pitch of the recording track,the recording track cannot be tracked correctly, with the result thatinformation signals cannot be recorded or reproduced on or from adesired recording track.

Thus, proposals have also been made for using an optical pickup deviceprovided with a light source designed for radiating a light beam of ashorter wavelength for reducing the spot size of the light beamilluminated on the magneto-optical disc. If the optical pickup device,radiating the light beam of the short wavelength, is used for recordingand/or reproducing a magneto-optical disc, the desired recording trackcannot be tracked correctly, because the spot diameter of the light beamis markedly smaller than the recording track width, with the result thatthe information signals cannot be recorded or reproduced correctly.

For differentiating the track pitch values of the recording tracks fromone another for ultimately enabling recording and/or reproduction ofplural sorts of the magneto-optical discs with different recordingdensities by a common optical disc recording and/or reproducingapparatus, there has also been proposed an optical disc recording and/orreproducing apparatus including an optical pickup device having a pluralnumber of light sources each radiating a light beam of a shorterwavelength and a light beam of a longer wavelength.

This optical disc recording and/or reproducing apparatus is designed toswitch between the plural light sources to radiate light beams ofdifferent wavelengths to adapt itself to a plural number of sorts of themagneto-optical discs having reciprocally different values of the trackpitch of the recording tracks.

With the magneto-optical disc having the track pitch of 1.6 μm, doublerefraction is more significant than with a magneto-optical disc having atrack pitch of 0.95 μm, and hence astigmatic aberration is produced inan optical system when the light beam is transmitted along an opticalpath in the magneto-optical disc. So, in the optical disc recordingand/or reproducing apparatus, dedicated to a magneto-optical disc havinga track pitch of 1.6 μm, the value of this astigmatic aberration issupervised with respect to the entire optical system. On the other hand,with the magneto-optical disc having the track pitch of 0.95 μm, thedouble refraction is lesser than with the magneto-optical disc having atrack pitch of 1.6 μm, such that the astigmatic aberration is suppressedin the optical system

[Problems to be Solved by the Invention]

With the above-described optical pickup device, having a plural numberof the light sources, the device itself is increased in size to renderit difficult to use the device in an optical disc recording and/orreproducing apparatus required to be reduced in size.

With the semiconductor laser, radiating a light beam of a shorterwavelength, such as approximately 650 nm, power consumption is largerthan with the semiconductor laser radiating a light beam of a longerwavelength, such as approximately 780 nm. The optical pickup device,employing a semiconductor laser with a larger power consumption, is notsuited to a battery-powered portable optical disc recording and/orreproducing apparatus. Additionally, with the semiconductor laser withlarge power consumption, the temperature coefficient is larger andundergoes significant self-heating, so that, when the semiconductorlaser is mounted on an optical pickup device, measures against heatradiation need to be taken to cause stabilized oscillations of the lightbeam when the semiconductor laser is mounted on the optical pickupdevice, with the result that it becomes difficult to reduce the size andthe thickness of the optical pickup device.

Moreover, the semiconductor laser radiating a light beam of a shorterwavelength is more expensive than the semiconductor laser adapted foroscillating the light beam with a wavelength of 780 nm, which has so farbeen used extensively, with the result that, if this semiconductor laseris used, it is not possible to reduce the cost of the optical pickupdevice and hence that of the recording and/or reproducing apparatus.

If, with the use of the optical disc recording and/or reproducingapparatus, dedicated to a magneto-optical disc having a track pitch of0.95 μm, information signals are recorded or reproduced on or from amagneto-optical disc having a track pitch of 1.6 μm, there is raised aproblem that astigmatic aberration is liable to be produced in therecording and/or reproducing optical system.

If, in a recording and/or reproducing apparatus dedicated to amagneto-optical disc having a track pitch of 0.95 μm, informationsignals are to be recorded or reproduced on or from a magneto-opticaldisc having the track pitch of 1.6 μm, there is raised a problem that adifference is produced, under the effect of the aforementionedastigmatic aberration, between an optimum point of the focusing bias indetecting the ADER (address in progressive error rate), as an error rateof the ADIP (address in pregroove), and an optimum point of the focusingbias of the RF signals of the magneto-optical disc having the trackpitch of 0.95 μm.

It is specifically necessary to electrically offset the focusing bias,optimized for recording and/or reproduction of information signals for amagneto-optical disc having a track pitch of 1.6 μm, with respect to thefocusing bias of the optical disc recording and/or reproducing apparatusoptimized for recording and/or reproducing information signals on orfrom a magneto-optical disc having a track pitch of 0.95 μm.

However, the optimum offset undergoes variations, due to the astigmaticaberration present in the individual recording and/or reproducingoptical systems, to render offset adjustment difficult.

There are also occasions wherein, depending on changes in shape of themagneto-optical disc or to tilt of the magneto-optical disc, the lightbeam is not incident at right angles to the recording surface of themagneto-optical disc. In this case, there is raised a problem that thelight beam incident on the recording surface of the magneto-optical discis not reflected in a direction perpendicular to the recording surfaceof the disc, such that the reflected light beam suffers coma aberrationto deteriorate the readout efficiency of the information signals.

It is therefore an object of the present invention to provide arecording and/or reproducing apparatus in which the apparatus may befurther reduced in size and thickness and in which information signalscan be recorded and/or reproduced on or from a magneto-optical dischaving a track pitch of 0.95 μm and a magneto-optical disc having atrack pitch of 1.6 μm. It is another object of the present invention toprovide a method for adjusting the aberration for such recording and/orreproducing apparatus

SUMMARY OF THE INVENTION

For accomplishing the above object, the optical disc recording and/orreproducing apparatus according to the present invention includes alight source for radiating a light beam having a wavelengthapproximately 780 nm, an objective lens having a numerical aperture NAof approximately 0.62 and adapted for converging the light beam radiatedfrom the light source to illuminate the optical disc, aberrationproducing means for producing the aberration in the light beamilluminated on the optical disc and light receiving means for receivingthe reflected light from the optical disc. Depending on the disc sort,as found by the disc discriminating means, the aberration producingmeans is driven to correct the aberration of different sorts produced inthe light beam to record and/or reproduce the information signals forthe optical disc.

The optical disc recording and/or reproducing apparatus according to thepresent invention records or reproduces information signals for pluralsorts of the optical discs, having different values of the track pitch,while correcting the astigmatic aberration and coma aberration producedin the optical system for optimizing recording and/or reproduction ofinformation signals for plural sorts of the optical disc with differentvalues of the track pitch.

The aberration adjustment method according to the present invention isused for an optical disc recording and/or reproducing apparatusincluding a light source for radiating a light beam having a wavelengthapproximately 780 nm, an objective lens having a numerical aperture NAof approximately 0.62 and adapted for converging the light beam radiatedfrom the light source to illuminate resulting converged light on theoptical disc, aberration producing means for producing the aberration inthe light beam illuminated on the optical disc and light receiving meansfor receiving the reflected light from the optical disc. The aberrationadjustment method includes a first aberration generation meansadjustment step of loading one of the plural optical discs withdifferent values of the track pitch of the recording track and hencedifferent values of the recording density, which has the smaller valueof the track pitch, on the apparatus, driving the aberration producingmeans, and correcting the coma aberration based on the informationsignals read out from the optical disc using light receiving means, anda second aberration generation means adjustment step of loading theoptical disc with the larger value of the track pitch, driving theaberration producing means, and correcting the coma aberration based onthe information signals read out from the optical disc using lightreceiving means.

With the aberration adjustment method of the present invention,information signals are recorded or reproduced for plural sorts of theoptical discs with different values of the track pitch, while theastigmatic aberration and coma aberration are corrected such as toprovide for optimized recording and/or reproduction of informationsignals for plural sorts of the optical disc having different values ofthe track pitch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a recording and/or reproducingapparatus to which the aberration adjustment method of the presentinvention is applied.

FIG. 2 is a schematic plan view showing a recording track of a firstmagneto-optical disc.

FIG. 3 is a schematic plan view showing a recording track of a secondmagneto-optical disc.

FIG. 4 is a schematic perspective view showing a liquid crystal deviceemployed in the present invention.

FIG. 5 is a schematic plan view showing each electrode pattern providedon a liquid crystal device.

FIG. 6 is a graph showing changes in the phase difference between apolarized component, having the same direction as the direction oforientation of liquid crystal molecules, of a light beam transmittedthrough the liquid crystal device, and a polarized component, having adirection perpendicular thereto, of the same light beam, with respect tothe driving voltage applied to the liquid crystal device.

FIG. 7 is a graph showing changes in jitter of read-out RF signals withrespect to the driving voltage applied to the liquid crystal device.

FIG. 8 is a graph showing changes in the error rate of RF signals withrespect to a driving voltage applied across first and second electrodepatterns and a common electrode pattern of the liquid crystal device.

FIG. 9 is a block diagram showing the structure of an aberrationadjustment device employed in the present invention.

FIG. 10 is a graph showing changes in ADER with respect to the drivingvoltage applied across the first electrode pattern and the commonelectrode pattern of the liquid crystal device.

FIG. 11 is a flowchart showing the aberration adjustment methodaccording to the present invention.

FIG. 12 is a schematic plan view showing another embodiment of eachelectrode pattern provided in the liquid crystal device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, preferred embodiments of the presentinvention will be explained in detail. It should be noted that thepresent invention is not limited to the embodiments now explained butmay be suitably modified without departing from its scope.

FIG. 1 shows an illustrative structure of an optical disc recordingand/or reproducing apparatus embodying the present invention.

The present optical disc recording and/or reproducing apparatus 1includes an optical pickup device 3 having an optical system, not shown,for illuminating a light beam on a magneto-optical disc 2, as arecording medium, and for detecting the light beam reflected from themagneto-optical disc 2, a magnetic head 4 for applying an externalmagnetic field modulated depending on the information signals to berecorded on the magneto-optical disc 2, and a spindle motor 5 carryingthe magneto-optical disc and adapted for operating as a rotatingmechanism for rotational driving this magneto-optical disc.

In the optical disc recording and/or reproducing apparatus 1, embodyingthe present invention, the magnetic head 4 is mounted facing the opticalpickup device 3, with the magneto-optical disc 2 in-between, and ismovable across the inner and outer rims of the magneto-optical disc 2 insynchronism with the movement of the optical pickup device 3.

The optical pickup device 3, thus provided on the optical disc recordingand/or reproducing apparatus 1, includes a semiconductor laser 11, as alight source for outputting a light beam illuminated on the signalrecording surface of the magneto-optical disc 2.

This semiconductor laser 11, radiating a light beam having a wavelengthapproximately equal to 780 nm, is widely used as a light source for anoptical pickup device used for reading in the information from anoptical disc having the track pitch of the recording track ofapproximately 1.6 μm.

The optical pickup device 3 includes, on one side of the semiconductorlaser 11, a grating 12 and a beam splitter 13 in this order looking fromthe semiconductor laser 11.

The grating 12 splits a light beam L1, radiated from the semiconductorlaser 11, into a main light beam and two subsidiary light beams, foracquiring tracking error signals by a three-beam method.

The beam splitter 13 separates the light beam illuminated on themagneto-optical disc 2 from the return light beam reflected from themagneto-optical disc 2. Meanwhile, a combination prism with a Wollastonprism is used herein as the beam splitter 13.

The optical pickup device 3 includes a collimator lens 14 forcollimating the light beam radiated at a preset radiation angle from thesemiconductor laser 11, and an objective lens 15 for converging thelight beam collimated by the lens 14 and for illuminating the collimatedlight on a signal recording surface of the magneto-optical disc 2, inthis order, when looking along a direction of transmission of the lightbeam from the semiconductor laser 11 from the beam splitter 13.

The optical pickup device 3 also includes an analyzer 16 for convertingthe Kerr rotation angle of the return light beam into output lightintensity, a multiple lens 17, and a photodetector 18, as lightreceiving means for receiving the light beam reflected back from themagneto-optical disc 2 and transmitted through the analyzer 16 and themultiple lens 17, in this order, when looking along the direction ofreflecting the return light beam of the beam splitter 13 reflected backfrom the magneto-optical disc 2.

Of these components, the photodetector 18 converts data recorded on themagneto-optical disc 2 into output electrical signals, depending onlight volume variations ascribable to the difference in the angle ofrotation of the plane of polarization of the received return light beamreflected back from the magneto-optical disc 2.

Meanwhile, such objective lens having the numerical aperture NA equal toapproximately 0.62 is used as the objective lens 15 adapted forconverging the light beam on the magneto-optical disc 2. The light beam,with the wavelength of 780 nm, transmitted through the objective lens 15for being converged on and illuminating the magneto-optical disc 2,forms a beam spot, with a spot diameter of approximately 1.53 μm, at thefocused position. That is, the light beam with the wavelength of 780 nmis converged by the objective lens 15, with the numerical aperture NA ofapproximately 0.62, and is illuminated at the focused position of theobjective lens 15 on the signal recording surface of the magnetic disc2, so as to form a beam spot with a spot diameter of approximately 1.53μm.

It should be noted that, in the magneto-optical disc 2 with a diameterof approximately 64 mm, a recording track 21 of a first magneto-opticaldisc 2 a, having a recording capacity of 140 MB, is formed with a trackpitch Tp₁ of approximately 1.6 μm, as shown in FIG. 2. This recordingtrack 21 includes a groove 21G, as a data recording area, and, on bothsides of the groove 21G, wobbled lands 21L for producing trackcontrolling and address detecting signals.

In the magneto-optical disc 2 with a diameter of approximately 64 mm, arecording track 22 of a second magneto-optical disc 2 b, having arecording capacity of 650 MB, is formed to have a track pitch Tp₂ ofapproximately 0.95 μm, as shown in FIG. 3. This recording track 22includes a data recording area as a land 22L, a groove 22G₁, on one sideof the land 22L, for delimiting the recording track 22, and a wobbledgroove 22G₂ for producing track controlling and address detectingsignals, on the opposite side of the land 22L.

It should be noted that, for optimally recording and/or reproducinginformation signals on or from a desired recording track of themagneto-optical disc 2, the light beam radiated from the optical pickupdevice 3 needs to correctly scan the recording track of themagneto-optical disc 2. In order for the light beam to correctly scanthe recording track of the magneto-optical disc 2, at least trackingcontrolling signals need to be generated so that the light beam scanningposition will be controlled based on this tracking controlling signal.That is, in order for the light beam to correctly scan the recordingtrack by the tracking controlling signals, the light beam needs to beilluminated on the entire width of the recording track to detect thewobbled lands 21L, 22L or the grooves 22G, 22G₁ and 22G₂.

With the optical disc recording and/or reproducing apparatus 1,information signals are recorded on or reproduced from the firstmagneto-optical disc 2 a having the track pitch of 1.6 μm and from thesecond magneto-optical disc 2 b having the track pitch of 0.95 μm, by asole light beam radiated from the semiconductor laser 11 as a sole lightsource.

So, with the optical disc recording and/or reproducing apparatus 1according to the present invention, a liquid crystal device 31 as meansfor generating the aberration is arranged between a collimator lens 14and an objective lens 15. This liquid crystal device 31 is used toadjust the astigmatic aberration and coma aberration to adjust the beamdiameter to record and/or reproduce information signals on or from thefirst magneto-optical disc 2 a having the track pitch of 1.6 μm and thesecond magneto-optical disc 2 b having the track pitch of 0.95 μm.

Specifically, the liquid crystal device 31 a first electrode plate 33and a second electrode plate 34 are arranged on both sides of a liquidcrystal plate 32, including liquid crystal molecules sealed therein, asshown in FIGS. 4 and 5.

Of these electrode plates, the first electrode plate 33 includes a pairof electrode patterns on both sides of a circular aperture 35,transmitting the light beam there through, as a reference. One of theseelectrode patterns is termed a first electrode pattern 36 a, with theother being termed a second electrode pattern 36 b.

On the first electrode plate 33, a pair of electrode patterns are formedon the outer sides of the first electrode pattern 36 a and the secondelectrode pattern 36 b. The electrode pattern formed on the outer sideof the first electrode pattern 36 a is a third electrode pattern 36 c,while the electrode pattern formed on the outer side of the secondelectrode pattern 36 b is termed a fourth electrode pattern 36 d.

Betwixt and around these first to fourth electrode patterns is formed anelectrode pattern termed a fifth electrode pattern 37.

On the second electrode plate 34 is formed a common electrode pattern 37facing these first to fifth electrode patterns 36 a to 36 e.

Across the first to fifth electrode patterns 36 a to 36 e and the commonelectrode pattern 37 is applied a driving voltage of the same ordifferent potential from a liquid crystal driving unit, not shown,connected to each electrode pattern. By the driving voltage beingapplied across the respective electrode patterns, the driving voltage isapplied to the liquid crystal plate 32 to change the orientation ofliquid crystal molecules in the liquid crystal plate 32.

Thus, with the present optical disc recording and/or reproducingapparatus 1, the beam diameter of the light beam radiated from thesemiconductor laser 11 may be adjusted by the liquid crystal device 31to effect optimum recording and/or reproduction of information signalson or from the first magneto-optical disc 2 a having the track pitch of1.6 μm and the second magneto-optical disc 2 b having the track pitch of0.95 μm.

In the liquid crystal device 31, the driving voltages V_(LC1), V_(LC2),V_(LC3), V_(LC4) and V_(LC5), applied across the fifth electrodepatterns 36 a to 36 e and the common electrode pattern 37, are appliedacross the respective electrode patterns, whereby the orientation of therespective liquid crystal molecules is changed responsive to therespective driving voltages V_(LC1) to V_(LC5). With the orientation ofthe liquid crystal molecules thus changed, phase difference is producedin the light beam traversing the liquid crystal device 31. This phasedifference, which depends on the respective driving voltages V_(LC1) toV_(LC5), applied to the liquid crystal device 31, is produced between apolarized light component having the same direction as the orientationof the liquid crystal device 31 and a polarized light component havingthe direction perpendicular thereto, and assumes locally differentvalues, for the respective electrode patterns, as a function of thedifferent values of the first to fifth driving voltages V_(LC1) toV_(LC5). The liquid crystal device 31 synthesizes the aforementionedphase difference to the astigmatic aberration and the coma aberration,produced in the optical system, to adjust the respective drivingvoltages V_(LC1) to V_(LC5) so that the RMS (root mean square) value ofthe wave front aberration of the beam spot, as throttled by theobjective lens 15, will be of the least value.

If, in the optical disc recording and/or reproducing apparatus 1,information signals are to be recorded on or reproduced from the firstmagneto-optical disc 2 a, with the track pitch of 1.6 μm, the drivingvoltages V_(LC1), V_(LC2) and V_(LC5), applied to the first, second andfifth electrode patterns 36 a, 36 b and 36 e of the liquid crystaldevice 31, respectively, are set at a certain equal potential, whilstthe driving voltages V_(LC3) and V_(LC4), applied to the third andfourth electrode patterns 36 c, 36 d, are set to the same potentialdifferent from the potential of the driving voltages V_(LC1), V_(LC2)and V_(LC5), thereby producing astigmatic aberration in the light beamto control the beam spot diameter to adjust the astigmatic aberration.

If information signals are recorded or reproduced on or from the secondmagneto-optical disc 2 b having the track pitch of 0.95 μm, therespective driving voltages V_(LC1) to V_(LC5), applied to therespective electrode patterns of the liquid crystal device 31, areadjusted so as not to produce astigmatic aberration in the light beam.The reason is that the second magneto-optical disc 2 b, having the trackpitch of 0.95 μm, undergoes double refraction only to a minor extent andhence is subjected to only little astigmatic aberration. Specifically,the driving voltages V_(LC1), V_(LC4) applied to the first and fourthelectrode patterns 36 a, 36 d and the driving voltages V_(LC2), V_(LC3)applied to the second and third electrode patterns 36 b, 36 c arerespectively shifted to the plus side and to the minus side of areference voltage, which will be explained in detail subsequently, toadjust the coma aberration.

Meanwhile, if the information signals are recorded on or reproduced fromthe second magneto-optical disc 2 b having the track pitch of 0.95 μm,the reference voltage is adjusted and set in advance so that the meanvalue of the driving voltages V_(LC1) to V_(LC5), applied to therespective electrode patterns and the common electrode pattern 37, willbe in the vicinity of 2/λ of the phase difference between a polarizingcomponent having the same direction as the orientation of the liquidcrystal molecules of the light beam transmitted through the liquidcrystal device 31 and a polarizing component having the directionperpendicular thereto.

FIG. 6 shows changes in the phase difference in case the respectivedriving voltages V_(LC1) to V_(LC5) to be applied to the respectiveelectrode patterns of the liquid crystal device 31, applied to therespective electrode patterns of the liquid crystal device 31, are setat the same potential, which same potential is changed so that theaforementioned phase difference will be in the vicinity of λ/2. FIG. 7shows changes in jitter of the read-out RF signals in case therespective driving voltages to be applied to the respective electrodepatterns V_(LC1) to V_(LC5), applied to the respective electrodepatterns of the liquid crystal device 31, are set at the same potential,which same potential is changed so that the aforementioned phasedifference will be in the vicinity of λ/2.

It may be seen from FIGS. 6 and 7 that, in case the respective electrodepatterns V_(LC1) to V_(LC5), applied to the respective electrodepatterns, are adjusted so that the aforementioned phase difference willbe close to λ/2, the jitter of the RF signals is reduced mostsignificantly in a manner most desirable in recording and/or reproducinginformation signals on or from the magneto-optical disc 2. This adjustedreference voltage is referred to below as the voltage V_(LC(ref)).

If the information signals are recorded or reproduced on or from thesecond magneto-optical disc 2 b having the track pitch of 0.95 μm, thereis no necessity for correcting the astigmatic aberration ascribable todouble refraction. However, the average voltage of the V_(LC1) toV_(LC5), applied to the respective electrode patterns of the liquidcrystal device 31 for correcting the coma aberration, is the voltageV_(LC(ref)) adjusted so that the aforementioned phase difference will beclose to λ/2. At this time, the driving voltages V_(LC1), V_(LC4)applied to the first and fourth electrode patterns 36 a, 36 d and thedriving voltages V_(LC2), V_(LC3) applied to the second and thirdelectrode patterns 36 b, 36 c are respectively shifted to the plus sideand to the minus side of a reference voltage, which will be explained indetail subsequently, to adjust the coma aberration.

The driving voltage V_(LC5), applied to the fifth electrode pattern 36e, is kept constant at the voltage V_(LC(ref)). It should be noted thatthe driving voltages V_(LC1) to V_(LC4) applied to the first to fourthelectrode patterns 36 a to 36 d are shifted so that the average value ofthe V_(LC1) to V_(LC5), applied to the respective electrode patterns,will remain at the voltage V_(LC(ref)), in such a manner that the jitterof the RF signals read out will be of the least value.

The driving voltages V_(LC1), V_(LC4), and the driving voltages V_(LC2),V_(LC3), adjusted as described above, are referred to below as thevoltage V_(LC(coma+)) and as the voltage V_(LC(coma−)), respectively.

If information signals are recorded on or reproduced from the firstmagneto-optical disc 2 a having the track pitch of 1.6 μm, the drivingvoltages V_(LC1), V_(LC2) and V_(LC5), applied across the first, secondand fifth electrode patterns 36 a, 36 b, 36 e and the common electrodepattern 37 are the aforementioned voltage V_(LC(ref)) which is to be thedriving voltage different from the driving voltages V_(LC3), V_(LC4)applied across the third and fourth electrode patterns 36 c, 36 d andthe common electrode pattern 37. That is, the driving voltages V_(LC3),V_(LC4) applied across the third and fourth electrode patterns 36 c, 36d, applied across the third and fourth electrode patterns 36 c, 36 d,are adjusted so as to be a voltage optimized for correcting theastigmatic aberration of the optical system.

FIG. 8 shows changes in ADER if, in case information signals are to berecorded or reproduced on or from the first magneto-optical disc 2 ahaving a track pitch of 1.6 μm, the driving voltages V_(LC1), V_(LC2)and V_(LC5), applied across the first, second and fifth electrodepatterns 36 a, 36 b, 36 e and the common electrode pattern 37, are fixedat the aforementioned voltage V_(LC(ref)), with the driving voltagesV_(LC3), V_(LC4) applied to the third and fourth electrode patterns 36c, 36 d of the liquid crystal device 31 being changed.

The ADER is now explained only briefly. In FIGS. 2 and 3, the areas A toD represent split areas of the light beam received by the photodetector18, with the photodetector 18 being split in register with these areas Ato D. The photodetector 18 calculates the level of the light beamreceived by its portions in register with the areas A to D. This levelis ADER. That is, ADER may be found by performing the calculations sothat, in case the disc is the first magneto-optical disc 2 a, the lightbeam level will be (A+D)−(B+C), or so that, if the disc is the secondmagneto-optical disc 2 b, the light beam level will be (A+B+C+D).

As may be seen from the graph of FIG. 9, the driving voltages V_(LC3)and V_(LC4) applied to the third and fourth electrode patterns 36 c, 36d are adjusted so as to be halfway in a area of the suppressed ADER.This adjusted voltage is VLC(AS).

Thus, if information signals are to be recorded on or reproduced fromthe first magneto-optical disc 2 a with the track pitch of 1.6 μm, thedriving voltages V_(LC3) and V_(LC4) applied to the third and fourthelectrode patterns 36 c, 36 d will be the adjusted voltage V_(LC(AS)).

The control of the various portions of the optical disc recording and/orreproducing apparatus 1 and the flow of various signals are nowexplained with reference to the block diagram of FIG. 9.

The optical disc recording and/or reproducing apparatus 1 includes amicro-computer 61 which is connected to a liquid crystal driving unit62, driving the liquid crystal device 31, a focusing driving unit 63 andto a track driving unit 64. The liquid crystal driving unit 62, focusingdriving unit 63 and the track driving unit 64 are connected to theoptical pickup device 3 and controls the driving of the optical pickupdevice 3 under a control signal from the micro-computer 61.

Based on the control signal from the micro-computer 61, the optical discrecording and/or reproducing apparatus 1 applies respective drivingvoltages L_(LC1) to V_(LC5) to the respective electrode patterns 36 a to36 e of the liquid crystal device 31. At this time, the micro-computer61 acts as disc discriminating means to decide the sort of themagneto-optical disc 2 to vary and adjust the respective drivingvoltages V_(LC1) to V_(LC5) to be applied to the respective electrodepatterns, depending on the sort of the magneto-optical disc 2.

Based on the control signal from the micro-computer 61, the optical discrecording and/or reproducing apparatus 1 applies the focusing bias tothe optical pickup device 3 to effect focusing servo.

It should be noted that the focusing bias is adjusted to be at a midportion of a voltage range for which RF signals read out from theoptical pickup device 3 can be optimally reproduced, that is for whichthe error rate is suppressed to a low value. This adjusted focusing biasis referred to below as focusing bias V_(FB).

The micro-computer 61 controls the track driving unit 64 so that theoptical pickup device 3 will correctly scan the recording track.

Referring to FIG. 8, the micro-computer 61 is connected to a rotationaldriving unit 65. Based on the control signal from the micro-computer 61,the rotational driving unit 65 controls the spindle motor 5 which thenruns the magneto-optical disc 2 in rotation at a preset rpm.

The optical pickup device 3 is connected to an RF amplifier 66, so thatRF signals reproduced by the optical pickup device 3 are sent to the RFamplifier 66 where the RF signals are amplified. The RF amplifier 66 isconnected to a DSP (digital signal processor) 67 and sends the RFsignals to the DSP 67 where the RF signals are converted into digitalsignals.

The DSP 67 is connected to the micro-computer 61 and to an ECC/ACIRC(Error Correction Code/Advanced Cross Interleaved Reed-Solomon Code)unit 68. From the DSP 67, the ADER, which is an error rate of ADIP, issent to the micro-computer 61, while EFM (Eight to Fourteen Modulation)signals are sent to the ECC/ACIRC unit 68.

The ECC/ACIRC unit 68 is connected micro-computer 61 and performsdecoding and error correction processing on the input EFM signals tosend the resulting error rate to the micro-computer 61.

The micro-computer 61 is connected to a RAM (Random Access Memory) 69,as storage means, and adjusts the liquid crystal driving unit 62,focusing driving unit 63 and the track driving unit 64 from variouserror information sent from the DSP 67 and the ECC/ACIRC unit 68 tostore adjusted optimum values of the voltages V_(LC(ref)), V_(LC(AS)),V_(LC(coma+)), V_(LC(coma−)) and the focusing bias _(VFB) in the RAM 69.Meanwhile, an EPROM (Erasable Programmable Read-Only Memory), forexample, may be used as RAM 69.

When the optical disc recording and/or reproducing apparatus 1 recordsor reproduces the information signals on or from the magneto-opticaldisc 2, the micro-computer 61 decides the type of the magneto-opticaldisc 2 by the micro-computer 61 to read out the voltages V_(LC(ref)),V_(LC(AS)), V_(LC(coma+)), V_(LC(coma−)) and the focusing bias V_(FB)from the micro-computer 61, as parameters, depending on the so decidedtype.

If specifically the disc is decided to be the first magneto-optical disc2 a having the track pitch of 1.6 μm, the voltage V_(LC(ref)) is readout as the driving voltages V_(LC1), V_(LC2) and V_(LC5), whilst thevoltage V_(LC(AS)) is read out as the driving voltages V_(LC3) andV_(LC4). If the disc is found by the micro-computer 61 to be the secondmagneto-optical disc 2 b having the track pitch of 0.95 μm, the voltageV_(LC(coma+)) is read out as the driving voltages V_(LC1), V_(LC4), thevoltage V_(LC(coma−)) is read out as the driving voltages V_(LC2),V_(LC3) and the voltage V_(LC(ref)) is read out as the driving voltageV_(LC5). As for the focusing bias, the focusing bias V_(FB) is read outfor any of the two magneto-optical discs.

The method for adjusting the aberration of the above-described opticaldisc recording and/or reproducing apparatus 1 is now explained withreference to FIG. 11.

First, at step S1, the second magneto-optical disc 2 b, with the trackpitch of 0.95 μm, is loaded on the optical disc recording and/orreproducing apparatus 1. Under the control signal from themicro-computer 61, the rotational driving unit 65 controls the spindlemotor 5 to run the second magneto-optical disc 2 b in rotation. Themicro-computer 61 decides the type of the second magneto-optical disc 2b.

Next, at step S2, the control signal from the micro-computer 61 controlsthe focusing driving unit 63, from which the focusing bias is applied tothe optical pickup device 3 to adjust the focal length of the objectivelens 15 with respect to the second magneto-optical disc 2 b to effectfocusing servo based on the error rate of the RF signals read out by theoptical pickup device 3. Here, as the driving voltages V_(LC1) toV_(LC5), applied to the respective electrode patterns of the liquidcrystal device 31, the voltage V_(LC(λ/2)), which is a voltageempirically obtained to give the aforementioned phase difference of λ/2,is used.

Next, at step S3, the micro-computer 61 verifies whether or not thefocusing bias is optimum, based on the error rate of the RF signal readout from the optical pickup 3. If the focusing bias is not optimum, theprogram reverts to step S2. If the focusing bias is optimum, the programmoves to step S4.

At step S4, the focusing bias, verified to be optimum by themicro-computer 61, is stored as the focusing bias in the RAM 69.

Next, at step S5, the liquid crystal driving unit 62 is controlled bythe control signal from the micro-computer 61. Based on the error rateof the RF signals, read out by the optical pickup device 3, the drivingvoltages V_(LC1) to V_(LC5), applied to the electrode patterns of theliquid crystal device 31, are adjusted so that the phase difference willbe in the vicinity of λ/2.

Next, at step S6, the micro-computer 61 verifies whether or not thephase difference is equivalent to λ/2, based on the error rate of the RFsignals read out by the optical pickup device 3. If the phase differenceis not equivalent to λ/2, the program reverts to step S5. On the otherhand, if the phase difference is equivalent to λ/2, the program moves tostep S7.

If, at step S7, the micro-computer 61 has verified that the phasedifference is equivalent to λ/2, the driving voltages V_(LC1) toV_(LC5), applied across thee respective electrode patterns of the liquidcrystal device 31, are stored in the RAM 69 as the voltage V_(LC(ref)).

Next, at step S8, the liquid crystal driving unit 62 is controlled bythe control signal from the micro-computer 61. Based on the error rateof the RF signals, read out by the optical pickup device 3, the drivingvoltages V_(LC1) and V_(LC4), applied to the first and fourth electrodepatterns 36 a, 36 d, respectively, and the driving voltages V_(LC2) andV_(LC3), applied to the second and third electrode patterns 36 b, 36 c,respectively, are shifted to the plus side and to the minus side,respectively, with the voltage V_(LC(ref)) as reference, so that theaverage driving voltage of the respective electrode patterns of theliquid crystal device 31 will be V_(LC(ref)), to adjust the shiftingquantity.

Then, at step S9, the micro-computer 61 verifies whether or not theaforementioned shifting quantity is optimum, based on the error rate ofthe RF signals read out by the optical pickup device 3. If the shiftingquantity is not optimum, the program reverts to step S8. If theaforementioned shifting quantity is optimum, the program moves to stepS10.

Then, at step S10, if the shifting quantity is verified by themicro-computer 61 to be optimum, the driving voltages V_(LC1) andV_(LC4), applied to the first and fourth electrode patterns 36 a, 36 d,respectively, and the driving voltages V_(LC2) and V_(LC3), applied tothe second and third electrode patterns 36 b, 36 c, respectively, arestored in the RAM 69 as the voltage V_(LC(coma+)) and as the voltageV_(LC(coma−)), respectively.

Next, at step S11, the second magneto-optical disc 2 b, having the trackpitch of 0.95 μm, is dismounted, whilst the first magneto-optical disc 2a, having the track pitch of 1.6 μm, is loaded in position. Undercontrol from the micro-computer 61, the rotational driving unit 65controls the spindle motor 5 to run the first magneto-optical disc 2 ain rotation. The micro-computer 61 verifies the type of the firstmagneto-optical disc 2 a.

Then, under control by the micro-computer 61, the liquid crystal drivingunit 62 at step S12 applies the voltage V_(LC(ref)) across the first,second and fifth electrode patterns 36 a, 36 b and 36 e of the liquidcrystal device 31 and the common electrode pattern 37, while applyingdriving voltages V_(LC3), V_(LC4) different from the voltage V_(LC(ref))across the third and fourth electrode patterns 36 c and 36 d and thecommon electrode pattern 37. It should be noted that the voltageV_(LC(ref)) is applied and fixed as the driving voltages V_(LC1),V_(LC2) and V_(LC5) across the first, second and fifth electrodepatterns 36 a, 36 b and 36 e of the liquid crystal device 31 and thecommon electrode pattern 37, whilst the driving voltages V_(LC3),V_(LC4), applied across the third and fourth electrode patterns 36 c and36 d and the common electrode pattern 37, are varied so that the ADERdetected from the RF signals read out by the optical pickup device 3will be optimum.

At step S13, the ADER is detected from the RF signals read out by theoptical pickup device 3, and the micro-computer 61 determines whether ornot this ADER is optimum. If the ADER is not optimum, the programreverts to step S12. If the ADER is optimum, the program moves to stepS14.

If the micro-computer 61 has verified at step S14 that the ADER isoptimum, the driving voltages V_(LC3), V_(LC4), applied to the third andfourth electrode patterns 36 c, 36 d of the liquid crystal device 31,are stored as the voltages V_(LC(AS)) in the RAM 69.

By the above procedure, the focusing bias V_(FB) and the voltagesV_(LC(ref)), V_(LC(AS)), V_(LC(coma+)) and V_(LC(coma−)) are stored inthe RAM 69. When recording or reproducing information signals on or fromthe magneto-optical disc 2 of the optical disc recording and/orreproducing apparatus 1, the above voltages may be used for correctingthe astigmatic aberration or coma aberration.

Meanwhile, it is assumed that, in adjusting the aberration of theoptical disc recording and/or reproducing apparatus 1, the voltageV_(LC(λ/2)) is pre-stored in the RAM 69.

The adjustment of the driving voltages V_(LC1) to V_(LC5) to be appliedto the respective electrode patterns of the liquid crystal device 31 ispreferably completed before shipment of the optical disc recordingand/or reproducing apparatus 1 as a product, whereby the adjustment ofthe recording and/or reproducing optical system of the individualoptical disc recording and/or reproducing apparatus 1 may befacilitated.

In the above-described embodiment, the first optical disc is loaded andadjusted after initially loading and adjusting the secondmagneto-optical disc. As an operating procedure, this process may bereversed, that is, the second optical disc may be loaded and adjustedafter initially loading and adjusting the first magneto-optical disc.

In this latter case, the same processing as that of steps S1 to S7 iscarried out for the first magneto-optical disc, such that the focusingbias V_(FB) and the reference voltage V_(LC(ref)) matched to the opticalproperties of the first magneto-optical disc are stored in the memory.

After storing the reference voltage V_(LC(ref)) in the memory,V_(LC(AS)) is adjusted and the second magneto-optical disc issubstituted for the first magneto-optical disc. Based on the referencevoltage V_(LC(ref)), the voltages V_(LC(coma+)) and V_(LC(coma−)) areadjusted and stored in the memory.

According to the present invention, as described above, informationsignals may be recorded on or reproduced from the magneto-optical discs2, having reciprocally different values of the track pitch of therecording track and hence different values of the recording density,with the aid of the liquid crystal device 31. Moreover, astigmaticaberration and coma aberration can be corrected extremely readily in arecording and/or reproducing optical system for magneto-optical discs.In addition, in the optical disc recording and/or reproducing apparatus1, since there is no necessity for the optical pickup device 3 to have aplural number of light sources, the device can be reduced in size,thickness and cost.

In a modification of the liquid crystal device, astigmatic aberrationmay be corrected using a generalized electrode patters, as shown in FIG.12. In this case, the astigmatic aberration may be corrected by applyingdifferent values of the driving voltage to two electrode patterns,namely a pair of crescent-shaped electrode patterns 71 on both sides ofan aperture 35 and an elliptically-shaped electrode pattern 72surrounded by the electrode patterns 71. However, in this case,correction means for correcting the coma aberration, such as liquidcrystal devices, are needed. Thus, the liquid crystal device 31, capableof correcting the astigmatic aberration and the coma aberration by thesole liquid crystal device, is more preferred in the perspective ofsimplifying the device structure and reducing the cost.

1. A method for adjusting an aberration in an optical disc device onwhich can be selectively loaded a first optical disc having a firstindex of double refraction or a second optical disc having a secondindex of refraction larger than said first index of double refraction,and which includes a liquid crystal device between a light source and anobjective lens converging a light beam radiated from said light sourceon the optical disc loaded on said optical disc device, said methodcomprising: an optimizing step of optimizing a focusing bias if theloaded optical disc is the second optical disc; a step of storing avalue of a focusing bias optimized at said optimizing step; a firstadjustment step of adjusting a voltage applied to said liquid crystaldevice so that the phase difference between a polarized light componentalong the arraying direction of said liquid crystal device and anotherpolarized light component along a direction perpendicular to saidarraying direction of said liquid crystal device will be approximatelyλ/2, where λ is the wavelength; a step of storing the voltage adjustedat said first adjustment step as a reference voltage in a memory; asecond adjustment step of adjusting the voltage applied to said liquidcrystal device for correcting an astigmatic aberration based on saidreference voltage; and a step of storing the voltage adjusted at saidsecond adjustment step as an astigmatic aberration correcting voltage inthe memory.
 2. The aberration adjustment method according to claim 1wherein, if the first disc is loaded after said second adjustment step,a coma aberration is corrected based on the reference voltage asadjusted at said first adjustment step.